docs: def do_tree(self, line):
          """Shows entities of a given kind."""
          opts = self.TREE_OPTS
          line = line.split()
          _pattern = ""
  
          if not self.current:
              self._help_noontology()
              return
  
          if len(line) == 0:
              # default contextual behaviour [2016-03-01]
              line = ["classes"]
  
          if line[0] not in opts:
              self.help_tree()
              return
  
          elif line[0] == "classes":
              g = self.current['graph']
              if g.all_classes:
                  g.printClassTree(showids=False, labels=False, showtype=True)
                  self._print("----------------", "TIP")
              else:
                  self._print("No classes available.")
  
          elif line[0] == "properties":
              g = self.current['graph']
              if g.all_properties:
                  g.printPropertyTree(showids=False, labels=False, showtype=True)
              else:
                  self._print("No properties available.")
  
          elif line[0] == "concepts":
              g = self.current['graph']
              if g.all_skos_concepts:
                  g.printSkosTree(showids=False, labels=False, showtype=True)
              else:
                  self._print("No concepts available.")
  
          else:  # should never arrive here
              pass